Screening a color cathode-ray tube

ABSTRACT

The mask/frame subassembly of a shadow mask type of color cathode-ray tube, having apertures of proper size for screening and having been oxidized to exhibit the heat-conducting and light-reflecting properties of a black body, is used as an exposure mask in photoprinting the dot triads of a mosaic-type screen for the tube. Thereafter, only the surface of the mask which faces the screen is subjected to a stripping step in which its oxide coating is removed. The mask/frame subassembly is now reetched so that the holes thereof are made larger than the elemental phosphor deposits of the screen, as required in black surround and post-deflection-focus color tubes.

United States Patent Inventor Josef Moegenbier Morton Grove, 111. 830,287

June 4, 1969 Sept. 14, 1971 Zenith Radio Corporation Chicago, Ill.

Appl. No. Filed Patented Assignee SCREENING A COLOR CATHODE-RAY TUBE 3 Claims, 3 Drawing Figs.

US. Cl 29/25.18, 29/25.1l, 96/361, 316/2 int. Cl 110119/16, H01 j 9/44 Field of Search 29/25. 1 25.1 1, 25.18; 316/2; 96/361 References Cited UNlTED STATES PATENTS 2,961,313 11/1960 Amdursky et a1 96/361 X 2,961,314 11/1960 Amdursky et al 96/361 X 3,070,441 12/1962 Schwartz 96/361 3,146,368 8/1964 Fiore et 111.... 96/361 X 3,231,380 1/1966 Law 96/361 3,351,996 11/1967 Fiore 29/2515 X 3,365,295 1/1968 Nettcl et a1. 96/84 Primary Examiner.l0hn F. Campbell Assistant Examiner-Richard Bernard Lazarus Att0rneyFrancis W. Crotty ABSTRACT: The mask/frame sub-assembly of a shadow mask type of color cathode-ray tube, having apertures of proper size for screening and having been oxidized to exhibit the heatconducting and light-reflecting properties of a black body, is used as an exposure mask in photoprinting the dot triads of a mosaic-type screen for the tube. Thereafter, only the surface of the mask which faces the screen is subjected to a stripping step in which its oxide coating is removed. The mask/frame subassembly is now reetched so that the holes thereof are made larger than the elemental phosphor deposits of the screen, as required in black surround and post-deflection-focus color tubes.

SCREENING A COLOR CATHODE-RAY runs RELATED APPLICATIONS The screening process of this invention is especially suited for use with that described and claimed in copending application Ser. No. 811,318, filed Mar. 28, 1969, in the name ofSam II. Kaplan and assigned to the assignee of this application.

BACKGROUND OF THE INVENTION There are certain improved types of color cathode ray tubes which impose an unconventional requirement as regards the relative sizes'of the elemental phosphor deposits and the transparent or aperture portions of the color selection electrode of the tube. For example, thereis described in U.S. Letters Pat. No. 3,146,368, issued Aug. 25, I964 and assigned to the assignee of the-present invention, a form of shadow mask color tube which distinguishes itself from conventional structures in that a pigment or light-absorbing material is deposited on the screen in the portions thereof that surround the elemental phosphor deposits and those deposits are made smaller in area than the electron transparent portions or apertures of the shadow mask of the tube'. For the dot triad type of screen used in the present day additive color system each such triad has a dot of green, a dot of blue and a dot of red phosphor and each of these dots is surrounded by a pigment. As explained in the reference patent, there are attractive resulting benefits, especially in enhancement of brightness and contrast. While a variety of pigments are suitable for use, a particularly attractive one is described and claimed in U.S. Letters Pat. 3,365,295, issued Jan. 23, 1968, and likewise assigned to the assignee of this invention. As there described, manganese carbonate is deposited around the multiplicity of phosphor deposits and, in the bakeout process of the tube a conversion is experienced in which the surround material converts to black manganese dioxide.

Another type of tube having the same general requirement that the apertures of the mask be larger than the elemental phosphor deposits is the post-deflection-focus tube in which the electron beams are subjected to an electron optical focusing system in order to assure that a larger percentage of the electron beams impinge upon the screen than is the case without post-deflection-focusing. While these two types of tubes have distinctly different attributes they do share the requirement that the phosphor deposits be smaller than the holes of the mask through which attributes selection is achieved in the operation of the tube.

A number of different processing techniques have been proposed for screening such a tube. An especially attractive one is the subject of application, Ser. No. 811,318, filed Mar. 28, 1969, in the name of Sam I-I. Kaplan. In screening, as taught in that application, the shadow mask which is normally developed by etching a blank, is prepared with apertures of the precise size required for screening. This has the advantage of permitting a higher degree of uniformity and a more precise control of hole diameter than is attainable in other procedures which feature forming the mask initially with its holes sizedas required in the final condition of the mask but temporarily filling or closing down those holes for the purpose of screening. After screening has been accomplished on the process of the above-identified Kaplan application, the mask is reeteched and the holes enlarged to the size desired of the mask for ultimate use in color selection in the tube. Where the practices of the Kaplan application are followed, highly satisfactory results are attained with short processing times, that is to say, the added steps of enlarging the mask holes after screening are easily an quickly accomplished. The present invention is a further development of that process directed to enhancing the mechanical strength of the mask/frame subassembly.

Accordingly, it is an object of the invention to provide a novel and improved method of screening a color cathode ray tube.

It is a specific object of the invention to improve the screening of a color tube whichfeatures reetching of the mask after the phosphor materials have been deposited.

SUMMARY OF THE INVENTION The invention is practiced in the screening of a color cathode ray tube of the type that has a color selection electrode which is oxidized on its opposed surfaces andiwhich has a pattern of apertures distributed therein. The invention is an improvement inthe general methods of screening wherein elemental areas of the screen of the-tube are exposed'to actinic energy projected onto the screen through the apertures of the color selection electrode. The improvement comprises stripping the oxide coating from onesurface only of the color selection electrode and then subjecting that electrode to an etchant, in which the material of the electrode is soluble, to enlarge the apertures thereof and establish a desired size of the enlarged apertures in relation to the size of the elemental areas of the screen that have previously been exposed and coated with phosphor.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a view showing the stripping step of the subject invention;

FIG. 2 is a fragmentary plan view taken as indicated by arrows 22 of FIG. 1; and

FIG. 3 is a fragmentary sectional view showing a mask/frame subassembly and a cooperating mask or shield.

DESCRIPTION OF THE PREFERRED EMBODIMENT In large measure the screening steps that are carried out in practicing the present invention are similar to those described and claimed in the above-identified Kaplan application. Accordingly, the illustrations of the drawing are confined to process steps that are unique tothe present invention.

Screening of the type under consideration is useful in any color cathode ray tube featuring a color selection electrode irrespective of such details as the size and shape of the envelope and the size and shape of its phosphor deposits. In particular it is of no consequence whether the tube be round or rectangular, nor is it of importance whether the phosphor deposits take the form of stripes or dots. For convenience, however, it will be assumed that the tube under consideration is rectangular and has a mosaic-type screen with a pattern of dot triads disposed over its image area. The envelope of such a tube conventionally is formed of two initially separate principal parts. There is a cap or faceplate section formed of the screen or image area with a circumscribing flange. The other part is generally conical with its large end dimensioned and configured to correspond with the flange of the faceplate so that they may readily be united through frit sealing. The smaller end of the conical section supports a neck which accommodates the usual cluster of three electron guns. The remainder of this disclosure will be confined to the screen sec tion and the method of processing it rather than the remaining structural parts of such a tube. v

As'indicated in FIG. 1, the tube under consideration for screening a mask/frame subassembly comprised of a mask 10 and a frame 11; a preferred form of this subassembly is the subject of U.S. Pat. No. 2,897,392, issued July 28, I959, in the name of Joseph P. Fiore. Its mask 10 is formed of a thin blank of annealed steel which may have a thickness of 5 mils. This blank has a multiplicity of apertures or holes distributed on accordance with the pattern desired for the distribution of triads on the screen of the tube. As initially formed, the size of the mask holes is that which is required for screening by photoprinting techniques and the holes may be of uniform size throughout the mask area or they may be weighted in accordance with known practices of the art, decreasing with radial displacement from the center to the edges of the mask. The field of apertures for the case under consideration is generally rectangular and it is bounded by a strip a of blank metal forming a flange to facilitate affixing to frame 11.

The frame is of heavier stock than the mask and may be stamped from cold rolled steel having a thickness of 93 mils. It is generally L-shaped in elemental cross section having as its peripheral element a portion 110, see FIG. 3, disposed essentially parallel to the axis of the tube when the mask is installed in position and having another part 11b that is generally normal thereto, being traversely disposed relative to the tube axis. The end surface defined by portion 11b of the frame is essentially planar but the other end is configured similar to the configuration of mask 10 so that these two components may be assembled in telescopic relation and attached to one another by welding as indicated at 12 in FIG. 1.

The mask/frame subassembly is oxidized to have the heatconducting and lightreflecting properties of a black body which is normally obtained by thermal blackening. The separate components 10 and 11 may be individually oxidized or the thermal blackening may take place after the subassembly has been made.

Of course, mounting springs are fixed to frame 11 so that the subassembly may be removably installed in operative position in the faceplate section of the tube. One such spring 13 is shown in FIG. I but there generally will be three or four depending on the choice that is made as between a three and a four point suspension. The leaf spring is affixed at one end to frame 11 and has an opening 13a at the opposite end to receive a mounting stud that projects inwardly from the flange of the faceplate in order to removably mount the mask/frame subassembly. An attractive method of processing this subassembly is the subject of U.S. Letters Pat. No. 3,351,996, issued Nov. I4, 1967, and assigned to the assignee of the present invention.

With the subassembly formed and oxidized as described, it is useful as a mask in screening especially where the screening involves photographic printing techniques. While a variety of such techniques are known, there is a distinct preference to photoresist or to electrostatic screening.

in photoresist screening, the faceplate is covered with a uniform layer of a slurry comprising a water-soluble resist and having one phosphor material in suspension. If the green phosphor is being processed, for example, the coating is a slurry containing that material and the mask/frame subassembly is installed within the faceplate after the coating has been applied and dried. The screen structure is then introduced into an exposure chamber or lighthouse where it is exposed with actinic energy, such as ultraviolet light, from a properly positioned source. For the green phosphor material under consideration, the source is positioned to simulate the electron gun of the tube that is to excite the green phosphor deposits. The ultraviolet light projected onto the screen through the shadow mask from that source exposes only those areas of the coating where deposits of green phosphor are desired. The exposure establishes a latent image of such green phosphor deposits which is then developed by washing the faceplate, usually after the mask has been removed, with deionized water. The developing step causes the screen to have dot type deposits of green phosphor arranged in a pattern corresponding to that of the pattern of holes in the shadow mask and precisely positioned on the screen area of the tube because the exposure was carried out with the shadow mask in its operative position.

In a generally similar sequence of steps, a slurry containing blue phosphor is next applied to the image area of the screen and is likewise exposed to ultraviolet light projected through the shadow mask but this time the light source is positioned to simulate the electron gun of the tube assigned to excite the blue phosphor deposits. Following the exposure, the blue deposits are developed. in like fashion, the deposits of red phosphor are applied to the screen, assuming of course that the light source is properly positioned to simulate the electron gun assigned to red for the tube in process.

The aforedescribed phosphor screening process is well understood in the art and it may take place either before or after the application of the light-absorbing material to the screen if the screen is to be of the type described and claimed in U.S. Letters Pat. No. 3,146,368. The present invention may be practiced whether or not the screen features the use of such a light-absorbing pigment and since the added processing steps required to apply such material to the screen are described in the last-mentioned patent, they will not be further described herein.

Electrostatic screening, as distinguished from slurry printing of the screen, has most desirable advantages from the standpoint of economy of materials and processing time. A disclosure of electrostatic screening, with representative formulations of the various materials employed, is found in copending application Ser. No. 481,316, filed Aug. 20, 1965, now U.S. Letters Pat. No. 3,475,169, issued Oct. 28, 1969 and likewise assigned to the assignee of the present invention. Briefly, a conductive layer is applied over the image area of the screen and then there is applied a superposed layer of a photocon ductor. This photoconductor is brought to a uniform level of charge by the use of a corona discharge device and thereafter elemental areas of the charged photoconductive layer are subjected to ultraviolet light that is projected onto the screen through the shadow mask. If the light source is positioned to simulate a particular electron gun of the tube, in much the same fashion as described in conjunction with slurry printing, the exposure develops a latent image of the elemental areas of the screen which are to receive a deposit of a particular phosphor material. That image is developed by flowing over the screen a toner which serves as a vehicle for carrying the phosphor in process. A surfactant may be used to polarize the toner and cause the phosphor material selectively to deposit in the charged areas of the screen that have been shielded against the exposure or, alternatively, in the discharged areas of the screen that have been exposed. The choice is determined whether one chooses to practice direct or inverse imaging as described in U.S. Letters Pat. No. 3,475,169. Here again, the same sequence of steps is carried out in applying each of the green, blue and red phosphor materials to form dot triads over the screen area.

Assuming that the screening has been completed, at least so far as the application of phosphors and light-absorbing pigment is concerned, the remaining process steps to achieve filming and aluminizing will be in accordance with prior art practices that need not be described. There are, however, important steps still to be taken in preparing the shadow mask subassembly for final installation within the tube after it has served its purpose in screening.

In particular, the invention comprises the step of stripping the oxide coating from but only one surface of the shadow mask preparatory to reetching for the purpose of enlarging the holes of the mask to a desired final size. To confine the stripping action to one surface of the mask, the remaining surfaces thereof are preferably protected or shielded against the stripping action. Either chemical or mechanical stripping may be practiced as desired. It has been found convenient to strip the oxide coating by the application of a stream of aluminum oxide or a sand blast using a mild abrasive carried in a stream under pressure. One may mount the mask/frame subassembly in a fixture 20 which preferably has recesses to accommodate mounting springs 13 and an elastic or pliable seat 20a upon which the subassembly may rest without being under stress. The subassembly may be clamped in position by a companion piece or ring 21 brought down over support 20 and configured to overlap the peripheral portion of the mask/frame subassembly. The clamp may have a soft lining 21a to protect the mask and may be moved into and out of clamping position manually or mechanically by a hydraulic system represented at 25.

FIG. 1 illustrates the mask/frame subassembly positioned within the stripping fixture 20, 21 which leaves exposed only the convex portion of mask which is the surface that is adjacent the screen area of the tube when the subassembly is in operative position with the faceplate. There is represented at 22 a nozzle for directing a stream of an abrasive against the exposed surface of the mask. This nozzle may be displaced, as indicated by the arrows, scanning it over the mask in order to strip the oxide coating from the entire part of the mask that contains the pattern of holes or apertures. In this process step, the ferric oxide coating formed in the thermal blackening process is removed to permit subsequent reetching without impairment by the oxide coating.

The subassembly may now be removed from fixture 20, 21 and be introduced into a reetching station where it is subject to an etchant in which the material of the mask is soluble. A suitable etchant is ferric chloride and etching may be accomplished by dipping, spraying, or any other convenient application of the etchant to the surface of the mask from which the oxide coating has been removed. The etchant is selected of appropriate concentration and the etching time is controlled so that the holes of the mask are enlarged to establish a predetermined size for the enlarged holes in relation to the phosphor deposits previously fonned on the screen of the tube. With the reetching completed and the holes enlarged to a final desired size, the mask/frame subassembly is again reoxidized to provide a black oxide coating on all its principal surfaces before final installation into the tube in process.

Very desirable advantages result from the described processing, especially from the process steps to which the mask/frame subassembly is subjected after the screening has been accomplished. These steps differ from the prior art in that the oxide is removed from one principal surface of the mask only, whereas in prior practices the entire subassembly was treated with a chemical stripper, such as hydrogen chloride, to remove the oxide coating from all surfaces of the subassembly. Where the oxide is removed from all of the principal surfaces, the etchant is free to attack all of those surfaces during the reetch process. With the described steps of the present invention, however, the etchant may attack the holes of the mask and one surface but not the other surface of the mask or the frame because the oxide coating that remains on these surfaces functions as a resist to the etchant. As the subassembly is formed, its component parts are subjected to cold rolling and acquire a certain surface strength in that process. This surface strength is largely preserved in practicing the present invention which exposes only one surface of the mask to the etchant during the reetch process whereas it is dissipated if the, oxide coating is removed from both surfaces of the mask. It will also be observed that the points of attachment [2 of the mask to the frame are similarly protected by an oxide coating which avoids the possibility of breaks at these points it they are subjected to the etchant. The weld points are localized areas of strain and would yield most quickly to attack by the etchant. This weakening of the structure is also avoided with the described process.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

1. in the screening of a color cathode ray tube of the type having a color selection electrode which is oxidized on its opposed surfaces and which has a pattern of apertures distributed therein and wherein elemental areas of the screen of the tube are exposed to actinic energy projected onto the screen through the apertures of the color selection electrode,

the improvement which comprises:

stripping the oxide coating from one surface only of said electrode;

and then subjecting said electrode to an etchant, in which the material of said electrode is soluble, to enlarge said apertures thereof and establish a predetermined size for the enlarged apertures relative to the size of said elemental areas of said screen.

2. The screening improvement in accordance with claim 1 in which the oxide coating is stripped from said one surface of said electrode by directing upon said surface a stream of an abrasive under pressure.

3. The screening improvement in accordance with claim 1 for a tube which further has a mounting frame to which peripheral portions of said electrode are attached by welding and which likewise is oxidized or its opposing surfaces, which improvement additionally includes the step of masking said frame and the points of attachment of said electrode thereto to protect them against the effect of said stripping step. 

2. The screening improvement in accordance with claim 1 in which the oxide coating is stripped from said one surface of said electrode by directing upon said surface a stream of an abrasive under pressure.
 3. The screening improvement in accordance with claim 1 for a tube which further has a mounting frame to which peripheral portions of said electrode are attached by welding and which likewise is oxidized or its opposing surfaces, which improvement additionally includes the step of masking said frame and the points of attachment of said electrode thereto to protect them against the effect of said stripping step. 